1. Technical Field
The present invention relates to an optical article with at least one side thereof facing the outside, such as a spectacle lens, and also to a method for producing the same.
2. Related Art
JP-A-9-113702 (Patent Document 1) describes provision of an optical article provided with a coating that has high environmental resistance and can be made transparent. For this purpose, a degeneration layer and a hard coating layer are formed on a transparent resin substrate by CVD; the degeneration layer contains at least one of Si and Ti and has a refractive index varying in the thickness direction, while the hard coating layer contains Si and O. The hard coating layer is described to have a thickness of not less than 0.4 μm and not more than 5 μm.
JP-A-7-56002 (Patent Document 2) describes uniform reduction of the reflectance of a plastic board over a wide range. It is described that for this purpose, when forming a hard coating layer on a plastic board to protect the plastic board in terms of strength, the hard coating layer is formed to have substantially the same refractive index as the refractive index of the plastic board at the portion where the hard coating layer comes in contact with the plastic board, and also that the refractive index of the hard coating layer varies continuously or gradually in the thickness direction.
Optical substrates made of plastic, such as plastic lenses, are lightweight as compared with glass lenses, which are one kind of optical substrates made of glass, and also have excellent formability, workability, and dyeing affinity, together with low breakability and high safety. Accordingly, they are widely used in the field of spectacle lenses. The plastic material (CR-39) developed by PPG Industries, U.S., in the 1940's has, as a spectacle lens material, an excellent balance of physical properties. Up to now, the plastic material (CR-39) has been long used as a material for plastic lenses. However, the refractive index thereof is as low as 1.50, and there are problems that in the case of lenses with minus power, such a lens has a large edge thickness, while in the case of plus lenses, the thickness at the center thereof is large.
In order to reduce the thickness of a plastic lens, the refractive index of the substrate has to be increased. The refractive index of plastic materials has been increased from 1.50 to 1.56, 1.60, 1.67, 1.70, 1.74, and 1.76, and thus materials with a high refractive index have been developed. Nowadays, plastic spectacle lenses of various refractive indexes are commercially available.
Optical substrates made of plastic are prone to scratches. To make up for this problem, a layer (film) called hard coating is often formed on the surface of the substrate to prevent scratches. The thickness of such a hard coating is approximately 1 to 5 μm. Materials typically used for forming the hard coating layer are thermosetting silicon-based resins and UV-curable organic resins. The refractive index of these materials is about 1.50 to about 1.55. When a thin layer is formed on a plastic substrate with a high refractive index (e.g., 1.60 or more), interference fringes are formed due to the difference in refractive index and nonuniformity in the thickness.
In a spectacle lens, such interference fringes will not affect the optical performance of the lens itself, but they degrade the appearance or cause reflected glare. Accordingly, in order to increase the commercial value, elimination of interference fringes is desired. In particular, although plastic lenses having a ultra-high refractive index (e.g., 1.70 or more) has a significant advantage of allowing production of thin, lightweight spectacle lenses, the resulting interference fringes are denser and more apparent.
One method for suppressing the formation of interference fringes is to form a hard coating layer containing a metal oxide for adjusting refractive index, so that the refractive indexes of the hard coating layer and the plastic lens substrate are comparable. For example, according to the above Patent Documents 1 and 2, the refractive index varies in the thickness direction so that there is no or little difference in refractive index at the portion where the hard coating layer and the plastics base material are in contact with each other, thereby reducing the reflectance. Further, the thickness is reduced so that the refractive index continuously varies. Accordingly, the refractive index of the surface is lowered, thereby reducing the reflectance. In this method, complete suppression of interference fringes requires a special hard coating to be designed, managed, and constructed for every substrate. However, for the production of coatings with various refractive indexes, a manufacturing facility is required for each case. This imposes a heavy burden on manufacturers.
In addition, although the original purpose of the hard coating layer is not to adjust the refractive index to suit a substrate, but to serve as a layer that imparts the substrate with weather resistance, abrasion resistance, and like various characteristics, as the refractive index of the layer increases, it becomes difficult to satisfy all these conditions.
Further, in many cases, an antireflection film is formed on the hard coating layer. When the refractive index of the hard coating layer differs, a special antireflection film also has to be designed for each case. Accordingly, even when the refractive index of a plastic lens is increased, thin, lightweight spectacle lenses and spectacles cannot necessarily be provided, and it is difficult to satisfy all the various quality characteristics.